build: establish Leaf build and release pipeline

src/main/java/games/dmg/leaf/LeafPlugin.java

@@ -0,0 +1,10 @@
+package games.dmg.leaf;
+
+import org.bukkit.plugin.java.JavaPlugin;
+
+public final class LeafPlugin extends JavaPlugin {
+    @Override
+    public void onEnable() {
+        getLogger().info("Leaf enabled.");
+    }
+}

src/main/resources/plugin.yml

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+name: Leaf
+version: ${version}
+main: games.dmg.leaf.LeafPlugin
+api-version: "1.20"
+description: Voluntary visible Resistance protection for non-aggressive players.
+author: dmg.games
+commands:
+  leaf:
+    description: Control or administer Leaf protection.
+    usage: /leaf <on|off|status|enabled|strength|player>
+permissions:
+  leaf.use:
+    description: Allows a player to control their own Leaf protection.
+    default: true
+  leaf.admin:
+    description: Allows administration of Leaf.
+    default: op

src/test/java/games/dmg/leaf/PluginMetadataTest.java

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+package games.dmg.leaf;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import java.io.InputStream;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+import org.yaml.snakeyaml.Yaml;
+
+final class PluginMetadataTest {
+    @Test
+    void declaresLeafEntrypointCommandAndPermissions() {
+        InputStream stream = getClass().getClassLoader().getResourceAsStream("plugin.yml");
+        assertNotNull(stream);
+
+        Map<?, ?> plugin = new Yaml().load(stream);
+        assertEquals("Leaf", plugin.get("name"));
+        assertEquals("games.dmg.leaf.LeafPlugin", plugin.get("main"));
+        assertEquals("1.20", plugin.get("api-version"));
+
+        Map<?, ?> commands = (Map<?, ?>) plugin.get("commands");
+        assertNotNull(commands.get("leaf"));
+
+        Map<?, ?> permissions = (Map<?, ?>) plugin.get("permissions");
+        Map<?, ?> usePermission = (Map<?, ?>) permissions.get("leaf.use");
+        assertEquals(true, usePermission.get("default"));
+        Map<?, ?> adminPermission = (Map<?, ?>) permissions.get("leaf.admin");
+        assertEquals("op", adminPermission.get("default"));
+    }
+}